A Highly Resilient Cantilever Spring Probe Having Curved Surfaces for Testing ICs

ABSTRACT

A probe has a cantilever arm coupled to a base via an anchor. A surface of the arm facing the base or a surface of the base facing the arm has steps with contact points that contact the base when the arm is depressed. Alternatively, the surface of the arm and/or the surface of the base may be curved.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of and incorporates by reference U.S.patent application Ser. No. 10/919,836 filed Aug. 16, 2004 by inventorPhillip Mai.

FIELD OF THE INVENTION

This invention relates in general to devices for testing electroniccircuitry and in particular, but not exclusively, to a device forprobing integrated circuits formed on a semiconducting wafer.

DESCRIPTION OF RELATED ART

Because many ICs (integrated circuits) formed on a semiconducting wafermay not be functional, it is desirable to test them before packaging orincluding them in a MCM (multi-chip module). The device which carriesthe electrical power and signals from a tester to the IC is known as aprobe card. Traditional probe cards carry the electrical signals to thebond pads of the IC via tapered needles which are affixed in acantilever manner to a PCB (printed circuit board). These needle probes,hereafter referred to as probes, are typically made byelectro-chemically etching a wire of tungsten, copper, palladium, oralloys thereof. During probing, the probes make physical contact withthe IC bond pads, scratching them in the process. Scratching isimportant in order for the probes to make good electrical contact withthe bond pads which are usually covered with a native and insulating,oxide film.

The total number of probes on a probe card has increased steadily as ICshave become more complex and multi-die testing has become the norm.Likewise, bond pad size and their pitch have also decreased toaccommodate the relatively smaller ICs. The aforementioned cantileverstyle probe cards do not scale well to the current trends primarilybecause they are manually intensive to build. Other horizontal probeshave been developed by borrowing heavily from semiconductormanufacturing. Eldridge, et. al. describes in U.S. Pat. No. 5,832,601 amethod by which a wire bonder makes the skeleton of the probe. Smith,et. al. describes a method of making a thin-film spring by sputtering inU.S. Pat. No. 5,613,861. Cohen describes a method by which 3-dimensionalstructures may be fabricated by the deposition and selective removal ofsacrificial metal layers in U.S. Pat. No. 6,027,630.

No known device, however, attempts to distribute the applied bendingstress uniformly along the length of the probe for the purpose ofgenerating more resiliency in a volume-limited space such as occurs inwafer-level testing.

SUMMARY OF THE INVENTION

The present invention is directed to a resilient spring probe whosedeflection is controlled in such a manner that the maximum bendingstress along the length of the spring when fully deflected is at orclose to the yield strength of the spring material.

In an embodiment of the invention, the probe comprises a cantileverspring arm; a base, coupled to the arm via an anchor at a first end ofthe arm; and a tip, disposed on a second end of the arm. A surface ofthe arm facing the base has a stepped surface. In another embodiment ofthe invention, a surface of the base facing the arm has a steppedsurface. In another embodiment of the invention, the surface of the armfacing the base and/or the surface of the base facing the arm is curved.

In an embodiment of the invention, a method comprises aligning a waferdie bond pad with a probe tip of the probe; driving the wafer into theprobe tip; and electrically testing the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a diagram illustrating a probe according to an embodiment ofthe present invention;

FIG. 2 is a diagram illustrating a perspective view of the probe FIG. 1;

FIG. 3 is a diagram illustrating a spring of FIG. 1 in a deflectedcondition;

FIG. 4 is a diagram illustrating a plurality of resilient springs bondedto a space transformer;

FIG. 5 is a diagram illustrating a probe according to another embodimentof the invention;

FIG. 6 is a diagram illustrating a probe with a conforming base/armpair; and

FIG. 7 is a flowchart illustrating a method of using a probe accordingto an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is provided to enable any person havingordinary skill in the art to make and use the invention, and is providedin the context of a particular application and its requirements. Variousmodifications to the embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles, features and teachingsdisclosed herein.

FIGS. 1 and 2 are diagrams illustrating an embodiment of the invention.Specifically, a cantilever spring-based probe 100 comprised of multiplelayers manufactured using electroplating technology discussed in U.S.Pat. No. 6,027,630 is illustrated. The probe 100 comprises an arm 110coupled to a base 140 via an anchor 150 at one end of the arm 110. A tip120 is disposed at the other end of the arm 110. The tip 120 is made ofa hard precious metal such as rhodium while the arm 110 is made from astrong conductive metal such as electrodeposited Ni. However, this doesnot preclude the use of other material combinations. The arm 110 furtherincludes a plurality of step like structures on both a top and bottomsurface. The steps on the bottom surface include contact points 130(e.g., 9) that contact the base 140 when the probe 100 is engaged. Inone embodiment, spacing between the contact points 130 decreases withdistance from the anchor 150.

In accordance with an embodiment of the invention the arm 110distributes the bending stress through the length of the probe 100 bycontrolling the deflection curve. As shown in FIG. 3, one or morecontact points 130 make contact with the base 140 as the arm 110deflects, thus preventing an overstress in the arm adjacent to thestructural anchor 150 while at the same time causing the arm's 110elastic energy to be stored further along the length of the arm 110. Aplurality of such probes 100 are shown bonded to a space transformer 200in FIG. 4. The space transformer 200 is in turn bonded to a PCB tocomprise the probe card.

Fully-stressed beams are known in the art. Textbooks on mechanicsdiscuss the parabolic-shaped cantilever beam as that which gives auniformly distributed maximum normal stress (i.e., ignoring shearstress). However, fully-stressed beams are normally associated withattempts to reduce weight, save material, and/or decrease moment ofinertia. None of these are a consideration for the probe 100.

Embodiments of the present invention are different from traditionalfully-stressed beams in three ways. First, it is not fully-stressed,only nearly so, because the arm 2 is comprised of flat layers and afinite number of contact points 130. Therefore, the bending stress willnot be perfectly uniform even though it is designed with uniformity inmind. Second, traditional fully-stressed beams vary their sectionmodulus for the above reasons, removing material where it's not needed.In the present invention the maximum stress uniformity is primarilyachieved by controlling the deflection curve and less so by varying thesection modulus. This translates into better resiliency and currentcarrying capacity because little material is removed from the arm 110just under the tip 120. The material is available to store elasticenergy and withstand electric current that might otherwise overheat athinned structure. Third, the present invention approximates thefully-stressed condition only at maximum overdrive. In the early stageof deflection the stress is concentrated at the structural anchor 150.In a traditional fully-stressed beam the stress is uniformly distributedeven at low stress.

The probe 100 dimensions can be varied to accommodate the requiredspring rate and maximum allowable deflection. However, there isn'tnecessarily a unique probe dimension to satisfy one particular springrate. Initial considerations determined by the physical probingrequirements might include allowable probe length, height, and width.Once these are decided, layer thicknesses and lengths can be optimizedfor the desired force or deflection using commercially available finiteelement analysis software. A typical probe would have an overall lengthof about 500-1000 microns, width of about 10-30 microns, and aninterlayer height of about 2-20 microns. The tip height would be about10-30 microns tall. These ranges are not meant to be exhaustive butmerely show that such a probe is well within current manufacturingcapabilities.

A further benefit of the contact points 130 is the reduced electricalpath that a signal must traverse from the tip 120. Instead of having totravel the full length of the arm 110 the signal would only have toshort directly into the base 140 via the contact points 130. Besidesimproving the high-speed performance by reducing the length ofuncontrolled impedance, the contact points 130 would also provide analternate parallel electrical path for higher current probes such aspower and ground probes. Joule heating would be reduced in the highlystressed areas thereby improving the probe's creep resistance.

Another embodiment of the invention also consisting of multiple layersmanufactured using technology discussed in U.S. Pat. No. 6,027,630 isshown in FIG. 5. In this embodiment a probe 300 comprises a base 350with a plurality of steps. The base 350 is coupled to an arm 320 via ananchor 330 at one end of the arm 320. A tip 310 is disposed at the otherend of the arm 320. The base 350 features a plurality of contact points340 along the steps of the base 350. The steps decrease in length asdistance from the anchor 330 increases. In an embodiment of theinvention, the arm 320 has a length and width equal to the length andwidth of the base 350. In an embodiment of the invention, the base 350includes 8 steps.

The arm 320 is of constant section modulus and the base 350 is steppedin order to control the deflection curve. As the probe 300 deflects, thecontact points 340 progressively make contact with the arm 320 thuspreventing an overstress in the arm adjacent to the structural anchor330 while at the same time causing the spring's elastic energy to bestored further along the length of the arm 320. In an embodiment theprobe tip 310 is made taller than the tip 120 by about 30-50 microns soas to prevent the fixed end of arm 320 from crashing into the waferduring overdrive. A taller tip produces a longer scrub in general. Thetip end 310 of the arm 320 rotates during overdrive, causing a tallertip to sweep out a longer arc. However, a longer scrub is not alwaysdesirable as it may overly damage the bond pad and cause wire bondreliability problems. In light of the above, it may be preferable to usethe probe 100, wherein the tip 110 height is minimized and the arm 110is stepped on the wafer side so as to prevent contact with the wafer.

In yet another embodiment of the invention, a multiple layer probe aspreviously described but with discrete contact points located on boththe arm 110 as in FIG. 1 and the base 350 as in FIG. 5.

It is within the scope of this invention that the discrete contactpoints either on the arm 110 or on the base 350 need not be the comersof steps but could be the surfaces of small protrusions.

A probe with discrete contact points along its length could bemanufactured laterally (in the width direction) in just one layer ratherthan vertically in multiple layers. A high aspect ratio lithographyprocess such as LIGA would be suitable for this purpose. In fact, with alaterally-built probe the control of the deflection curve need not bewith discrete points but rather with a continuous base/arm conformingpair. An embodiment is illustrated in FIG. 6. Upon deflection the armconforms to the shape of the base in a continuous manner. As with thediscretely contacting probe, there is no unique base/arm shape for adesired spring rate, and finite element analysis software can aid in adesign that distributes the bending stress along the length of the arm.While not preferred due to the difficulty in handling and assemblingindividual probes into a larger array, such laterally-built probes,manufactured in one or more layers, are within the scope of theinvention.

It is also within the scope of this invention that the spring probe hasa residual stress so as to further improve the resiliency. The residualstress may be induced by initially overdriving the probe beyond itsas-deposited yield strength so that upon elastic relaxation bothcompressive and tensile residual stresses exist within the body of theprobe. Overstressing the spring serves two functions. First, it strainhardens the as-deposited material in the outermost sections of the beam(i.e. that part of the beam farthest from the neutral axis) therebyincreasing the yield strength in that part of the beam. Second, itimparts a residual stress field that improves the resiliency byredistributing the elastic strain energy during bending from the outersections of the beam to the interior sections. The discrete contactpoints must be adjusted in order to provide a uniform bending stressalong the length of the beam that exceeds the as-deposited yieldstrength of the material. The spring probe will be deformed from itsas-deposited shape, but resiliency for future overdrives will beimproved due to the strain hardening effect and residual stress field.

The manufacturing process is a layer by layer process so the arm andbase will be parallel to each other in the as-deposited condition. Whenthe arm is overdriven so as to create a residual stress, the arm is atan angle to the base. The angle that the arm makes with the base changesgradually along the length of the probe until it reaches a maximum atthe tip. The angle the tip rotates from vertical due to the preloadingcan vary from about 5 to about 15 degrees.

FIG. 7 is a flowchart illustrating a method 700 of using a probeaccording to an embodiment of the present invention. First, a probe cardhaving at least one probe (e.g., probes 100 and/or 300) is loaded (710)into a prober tray of a probing device. Next, a tester interface isconnected (720) to a probe card PCB. Wafer die bond pads are thenaligned (730) with the probe tips of the probes using a prober. Thewafer is then driven (740) into the probe tips and electrically tested(750). After testing, if (760) there are no more dies to test, then themethod 700 ends. Otherwise, the method 700 further comprises stepping(770) to the next die so that its bond pads are beneath the probe tipsand the driving (740) and testing (750) are repeated until all die onthe wafer are tested. The method 700 then ends.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

1. A probe, comprising: a cantilever spring arm; a base, coupled to thearm via an anchor at a first end of the arm; and a tip, disposed on asecond end of the arm; wherein at least one of a surface of the armfacing the base and a surface of the base facing the arm is curved. 2.The probe of claim 1, wherein the surface of the arm facing the base iscurved.
 3. The probe of claim 1, wherein the surface of the base facingthe arm is curved.
 4. The probe of claim 1, wherein both surfaces arecurved.
 5. The probe of claim 1, wherein the surface not curved has astepped surface.
 6. A probe card, comprising: a space transformer; aplurality of probes coupled to a surface of the space transformer; and aprinted circuit board communicatively coupled to the space transformer;wherein the probes each comprise a cantilever spring arm; a base,coupled to the arm via an anchor at a first end of the arm; and a tip,disposed on a second end of the arm; wherein at least one of a surfaceof the arm facing the base and a surface of the base facing the arm iscurved.
 7. A method, comprising: aligning a wafer die bond pad with aprobe tip of the probe of claim 1; driving the wafer into the probe tip;and electrically testing the wafer.